New approaches in microscopy and fluorescent analog cytochemistry have enabled the trafficking of fluorescent molecules to be studied in real time within living cells (1). The value of such approaches have been greatly extended by producing protein analogs which report changes in their structure or ligand binding through changes in their fluorescence spectrum. This has been accomplished through site-specific protein labeling with environmentally sensitive fluorophores that report changes in their solvent environment, or through labeling with pairs of dyes undergoing fluorescence resonance energy transfer. Using such fluorescent protein biosensors, the spatio-temporal dynamics of protein localization have been correlated with alterations in protein-protein interaction and conformation. This core will supply the resources and expertise to carry out quantitative studies of fluorescent molecules in living cells. Well characterized fluorescent molecules suitable for examining trafficking will be supplied, as well as fluorophores and help for generating novel fluorescent analogs. The core director is actively involved in development of protein analogs which report conformation changes and ligand interactions in vivo (2). The core will provide the novel dyes required for such studies and assist investigators in producing new conformationally-sensitive protein biosensors.